1. Field of the Invention
This invention relates to electrode sealing arrangements for electric arc furnaces.
2. Review of the Art
Electric arc furnaces usually comprise a crucible and generally vertical carbon electrodes supported so as to depend into the crucible. In order to contain fumes, maintain a desired atmosphere within the furnace and control heat loss, it is common practice to provide such furnace with a lid or roof having apertures through which the electrodes depend. In furnaces where it is necessary to open the lid or roof, it is necessary to withdraw the electrodes, which will be extremely hot at their lower ends. Depending on the type and usage of the furnace, a greater or lesser amount of vertical electrode movement must be accommodated during normal operation of the furnace, and electrodes must also be advanced downwardly to compensate for consumption of the electrode tips. Additionally, and particularly during the initial stages of melting a charge, current surges through the electrodes apply very substantial electromagnetic forces to them, which can cause significant lateral deflection. These and other factors place extremely severe requirements upon arrangements designed to provide a seal between the electrodes and roof of an electric furnace.
Where only a moderate range of electrode movement needs to be accommodated, water cooled telescoping seals may be utilized, as exemplified by those described in French Patent No. 1,418,153 (IRSID), U.S. Pat. No. 4,306,726 (Lefebvre) and U.S. Pat. No. 4,027,095 (Kishida et al).
U.S. Pat. No. 4,745,619 (Strobele) discloses an electrode seal structure in which a water cooled sleeve is provided around that portion of the electrode extending through the roof during normal operation of the furnace, supporting or providing a seal element which interacts with a further annular sealing element retained around the associated aperture in the furnace roof such that the electrode mounted sealing element may be lifted clear of the sealing element retained on the furnace roof when electrodes are withdrawn from the furnace to permit opening of the roof. In such an arrangement, the water cooled sleeve complicates the electrode structure, and even if carefully designed may be vulnerable in some cases to damage by arcing within the furnace when at the lower end of its range of movement. Other seal structures utilizing water cooled sleeves surrounding and carried with the electrodes have been proposed in U.S. Pat. No. 1,690,795 (Sagramoso) and U.S. Pat. No. 4,347,400 (Lamarque).
Proposals have also been made for electrode seal structures which are supported by the furnace roof and act directly upon the surface of the electrode in sliding contact with it. U.S. Pat. No. 4,457,002 utilizes a water cooled sleeve depending into the furnace, but supported from the furnace roof. Such an arrangement has many of the same disadvantages as arrangements using a sleeve attached to the electrode, requires a multi-segmental shield ring below the sleeve, and provides only limited clearance to accommodate lateral movement of the electrode. It does not appear suitable for applications in which the electrodes must be frequently withdrawn through the roof of the furnace, as for example in furnaces for melting scrap steel.
U.S. Pat. No. 4,442,526 (Rappinger) discloses an electrode seal for use in conjunction with a furnace having a conductive water cooled roof. Very little detail is provided of the structure Of the seal itself, and the entire arrangement appears predicated upon the use of a water cooled roof.
U.S. Pat. No. 3,838,233 (Prenn) discloses a system for supporting the weight of seal assemblies independently of a furnace roof, whilst permitting accommodation of lateral movement of the electrodes. The seals themselves are rings of refractory materials, possibly supplemental by pivoted segmented seals. If such pivoted seals are used, it is not apparent how adequate electrical isolation is maintained between the electrodes.
U.S. Pat. No. 4,295,001 (Britton) and U.S. Pat. No. 4,759,032 (Willis) both disclose arrangements in which a large number of refractory seal segments are pressed inwardly against the electrode or each electrode so as to maintain a desired seal.
U.S. Pat. No. 2,979,550 (Sherman) discloses an arrangement in which a large number of pivoted, segmented, water cooled seal segments are pressed inwardly by gravity and spring action against an electrode, a supplementary diaphragm seal being provided between the segments and a support ring.
In practice, none of the above systems has proved generally acceptable to operators of arc furnaces used for the melting of steel. One solution that has been proposed is a form of gas seal, in which compressed air is blown into the gaps between the electrodes and the furnace roof, as described in an article "Preventing EAF Fume and Dust Escape" by Mark Cywinski, Steel Times International, November 1992. This solution of itself represents a tacit acknowledgment that satisfactory electrode seal arrangement is not generally available for use in steel melting applications.
An object of the present invention is to provide an electrode seal for electric arc furnaces which is effective, of relatively simple and economical construction, which can be made sufficiently light to be safely supported on a refractory furnace roof, which is compatible with repeated complete withdrawal of the electrodes from the furnace, and which can help extend the life of the refractory roof.
Rather than trying to force seal elements against the surface of an electrode so as to maintain a seal, which tends to result in excessive wear on the seal elements and may incur the risk of jamming, I form a sealing ring in a relatively small number of water cooled metallic segments, resiliently urged into circumferential end-to-end abutment so as to form a close-fitting girdle around the electrode having an internal diameter which closely matches the nominal external diameter of the electrode. Electrode manufacturers specify the outside diameter of carbon electrodes within comparatively narrow tolerances, but in practice much closer tolerances are normally maintained. By setting the internal diameter of the sealing ring in a suitable relationship to this narrower range of tolerance, a good fit can be maintained, whilst excessive wear or frictional engagement is avoided because inward movement of the seal segments is limited by their end-to-end abutment. Present indications are that an internal diameter near the upper end of the range of diameters within which electrode diameters normally fall will provide the best results, but experience may suggest some upward or downward adjustment of this relationship. The objective is to provide an arrangement in which the ring segments will normally abut or very nearly abut, and separation of the segments will normally be needed only to accommodate adherents to the surface of the electrode and irregularities at joints between electrode segments. Any displacement against the resilient bias which is needed to accommodate the electrode itself will be very small indeed. The sealing ring is resiliently pressed downwards so as to form a planar annular seal with a planar surface on a further seal component which also provides a cylindrical water cooled liner for the aperture in the furnace roof through which the electrode passes. This further seal component not only completes the seal whilst allowing for lateral movement of the electrode, but also anchors the assembly to the roof and protects the inside surface of the aperture against the thermal stresses to which it would otherwise be subject.
Further features of the invention will be apparent from the following description of a presently preferred embodiment and from the appended claims.